In restorative dentistry, high-performance ceramics such as Al2O3 and tetragonal ZrO2 have become very important for the manufacture of wide-span bridges and high-load-bearing crowns. The emphasis will be on the processing of ZrO2 materials below. These dental restorations are preferably prepared by machine-processing of ZrO2 blanks which are produced from granulated ZrO2 powders by pressing followed in most cases by thermal treatment.
Various methods for colouring oxide-ceramic powder such that the moulding prepared therefrom has the desired colour are known from the state of the art.
The best-known method is the admixing of colouring oxides into the granular material of the oxide ceramic. After it is pressed, processed and thermally treated, the finished coloured moulding then forms. As a result of the thermal treatment, the colouring ions cover a lattice site or an interstitial site. The most important documents for this are: U.S. Pat. Nos. 5,219,805 or 5,263,858, 5,656,564 or 5,059,562 or 5,118,457.
Further approaches have been proposed for the preparation of coloured ceramic mouldings, in particular dental mouldings, the colouring of which comes as close as possible to that of natural teeth. Thus processes are known from the state of the art according to which coloured blanks or also dental restoration parts are obtained by infiltrating liquids into a presintered moulding (U.S. Pat. No. 6,709,694 and EP 1 486 476 respectively). However, these processes have the disadvantage that colouring takes place after the presintering process and thus liquids are introduced into an open-pored ceramic body. Thus the colouring is not completely homogeneous. Nor can polychromatism be achieved. In particular, by subsequently colouring a partly blank or a shaped dental product, it can be that only the cavities (pores) between the partially co-sintered particles of the starting powder are covered by the colouring materials. Thus also only discrete areas of the surface of the particles are coloured with a layer of the colouring oxides, but a continuous covering of the surface of the particles of the starting powder is not possible. A further great disadvantage with an infiltration is the concentration gradient of the colouring from the outside in. If a porous body is introduced into the colouring solution, the starting solution releases some of the dissolved colouring ions on entering the body and is thus “depleted” of colouring substances from the outside in. Consequently there is a higher concentration of the colouring ions and oxides outside than inside the moulding. Furthermore only a certain penetration depth can be achieved by means of the infiltration technique.
EP 1 859 757 A2 describes a process for the preparation of coloured blanks and dental mouldings. Oxide-ceramic powders in granular form are coated with a chromophoric substance in aqueous solution in a fluid-bed reactor. The thus-obtained coated granular material is then pressed to form a moulding which can be further processed to form a dental spacer after a presintering by milling or grinding.
The mouldings prepared by compression processes are usually obtained in the form of block bodies from which the desired restorations can be milled, for example using CAD/CAM. Such block bodies, even with a continuous colouring, are described for example in EP 1 859 758 A1. The disadvantage of this technique is in particular the considerable loss of valuable sinter ceramic associated with the milling process.
In addition to the dry-pressing processes, the use of ceramic particles in suspensions, in particular in the form of so-called slips, is also known in the state of the art. It can be advantageous here to apply to the ceramic particles a coating which can serve for example as a processing aid. Thus DE 10 2005 003 755 A1 describes a process for coating a dental powder with inorganic substances such as Bronsted or Lewis acids or bases, or organic substances, in particular certain polymers. The coating takes place in each case using an aqueous solution of the coating component either directly by introduction of the powder into the aqueous medium or by a fluid-bed process.
Furthermore, a process is known from EP 1 210 054 according to which the coloured blanks are prepared from partially stabilized zirconium dioxide such that the starting materials are dissolved in water in the form of their soluble chlorides, including the colouring substances, a co-precipitation is carried out and the precipitation product is calcined at approx. 700° C. After the grinding of the calcinate and a spray-drying, the thus-obtained granular material is isostatically pressed and then thermally treated (debinding and presintering).
In dental engineering, the most common method to date is the grinding or milling of blocks, plates or cylinders. The decision as to which type of mechanical processing is chosen depends on the respective machines, but also on the state of the oxide ceramic (ratio of the density of the part to be processed to the theoretically achievable density).
The so-called constructive or generative manufacturing processes represent a further approach to the formation of oxide-ceramic mouldings. The term “Rapid Prototyping” (RP) covers generative manufacturing processes in which 3-dimensional models or components are prepared from computer-aided design data (CAD data) (A. Gebhardt, Vision of Rapid Prototyping, Ber. DGK 83 (2006) 7-12). These are processes such as e.g. stereolithography (SL), selective laser sintering (SLS), 3D printing, fused deposition modelling (FDM), ink-jet printing (IJP), 3-D plotting, multi-jet modelling (MJM), solid freeform fabrication (SFF), laminated object manufacturing (LOM), laser powder forming (LPF) and direct ceramic jet printing (DCJP), with which models, components or mouldings can be prepared cheaply even on a small scale (A. Gebhardt, Generative Fertigungsverfahren, 3rd ed., Carl Hanser Verlag, Munich 2007, 77 et seq.). Stereolithography involves RP processes (A. Beil, Fertigung von Mikro-Bauteilen mittels Stereolithographie, Düsseldorf 2002, VDI-Verlag 3 et seq.) in which a mouldings is constructed in layers from a liquid and curable monomer resin on the basis of CAD data.
RP processes for the preparation of dental mouldings such as inlays, crowns or bridges are highly advantageous particularly with ceramic materials, because the impression-taking and casting processes and the grinding and milling operations respectively, which involve considerable manual outlay in the dental engineering laboratory, can thus be greatly simplified and at the same time the material loss which occurs with non-generative processes can be avoided. As a complete digital process chain is in place today, the standard process steps for the preparation of e.g. multi-unit bridge frameworks (alignment in the articulator, wax modulation, embedding and casting) can be replaced by the digitalization of the model, virtual design of the dental moulding and its generative stereolithographic manufacture.
Both stereolithography and increasingly also 3D printing have proved to be important methods for preparing dental mouldings from oxide-ceramic materials. Sprayable ceramic inks which contain oxide-ceramic particles in a cross-linkable solvent are used for 3D printing. After a layer has been deposited, curing takes place by high-energy radiation. In contrast, in stereolithography a layer of a cross-linkable slip is cured by targeted illumination.
The composition of the slips and of the ceramic inks are substantially the same, at least in respect of the components used. These are in both cases the oxide-ceramic particles, a cross-linkable monomer or monomer mixture, an initiator or initiator system and optionally further auxiliaries such as solvents etc.
In the preparation of ceramic mouldings e.g. by means of stereolithography, a ceramic green compact is firstly prepared by layered curing of a free-flowing ceramic slip which is then sintered after debinding to form a dense ceramic moulding. The green compact is also called a green body. The term debinding is used to describe the elimination of the binder. Here, the binder used is usually removed by heating the green compact to a temperature of approx. 90° C. to 600° C. It is essential that the formation of cracks and deformations is very largely avoided. The green compact becomes the so-called white body as a result of the debinding.
In debinding, purely thermal and thermochemical processes take place. Mixtures of water, solvents, polymers, waxes or oils are usually used as binders in the pressing of ceramic powders. Polypropylene, polyethylene, polyvinyl acetate, polyvinyl alcohol, methylcellulose, polyvinylpyrrolidone, polystyrene or polyethyl methacrylate are mostly used as polymers (cf. R. Moreno, Amer. Cer. Soc. Bull. 71 (1992) 1647-1657). These are linear polymers which are broken down more or less easily into volatile components by depolymerization or chain-splitting at increased temperature.
In the case of green bodies produced by RP processes based on cross-linking monomer mixtures, there is a polymer network. Through the use of cross-linking monomers the curing time which is required to obtain a stable solid can be significantly shortened, but at the same time the polymer network that forms also displays a much higher thermal stability compared with linear polymers, which adversely affects the debinding process.
The sintering of the white body takes place in the sintering furnace during high-temperature firing. The finely-distributed ceramic powder is compacted and solidified by exposure to temperature below the melting temperature of the main component, as a result of which the porous component becomes smaller and its strength increases.
U.S. Pat. No. 5,496,682 discloses light-curable compositions for the preparation of three-dimensional bodies by stereolithography, which contain 40 to 70 vol.-% ceramic or metal particles, 10 to 35 wt.-% monomer, 1 to 10 wt.-% photoinitiator, 1 to 10 wt.-% dispersant and preferably also solvent, plasticizer and coupling agent.
DE 10 2006 015 014 A1 describes a process for the preparation of three-dimensional ceramic mouldings by layered imprinting of a suspension with the help of an ink-jet printer. The suspension contains ceramic particles in a dispersant medium based on an aqueous boehmite sol.
U.S. Pat. No. 6,117,612 discloses resins for the stereolithographic preparation of sintered ceramic or metal parts. The resins have a viscosity of less than 3000 mPa·s. For their preparation, monomers with a low viscosity are used, preferably in aqueous solution. A high solids content and low viscosity are said to be achieved through the use of dispersants.
DE 199 50 284 A1 describes compositions curable with visible light based on polymerizable monomers or oligomers and their use for the preparation of dental restorations made of plastic materials with RP processes.
A particular problem in the preparation of ceramic spacers by RP processes is the colouring of the ceramic, as the colorants used must survive the debinding and sintering process. Moreover, it has been shown that when using pigments, i.e. predominantly crystalline inorganic substances, to colour ceramics, patchy accumulations of pigments often occur in the ceramic. As a result of this inhomogeneous distribution of the pigments, for one thing the desired colour effect is not achieved, but in addition the translucence of the ceramic is also impaired. In addition, the high local concentration of foreign material in the ceramic often results in a reduction in strength.
The processes described above are in particular not sufficiently suitable to construct, using generative processes, coloured ceramic mouldings which satisfy the demands made of dental materials. The object of the invention is therefore to provide an improved technique for the preparation of coloured ceramic mouldings in particular by means of RP processes.